[MacDorcha]

what is the universe expanding into?

[LibWhacker]

what is the universe expanding into?

There is nothing "outside" of the universe for the universe to "expand into."

[Godebert]

"what is the universe expanding into?"

That's what I'm wondering. Is it possible that there are many other universes beyond ours that cannot be seen. Could all these universes be some sort of super-sub-atomic particles in an even bigger universe?

[Sentis]

what is the universe expanding into?

It is expanding into what Michio Kaku calls Hyperspace. This is basically a higher dimensional space which can only be seen at the quantum level.